Field of the Invention
The invention relates generally to a current-perpendicular-to-the-plane (CPP) magnetoresistive (MR) sensor that operates with the sense current directed perpendicularly to the planes of the layers making up the sensor stack, and more particularly to a CPP sensor with an improved ferromagnetic free layer structure.
Background of the Invention
One type of conventional magnetoresistive (MR) sensor used as the read head in magnetic recording disk drives is a “spin-valve” (SV) sensor. A SV MR sensor has a stack of layers that includes two ferromagnetic layers separated by a nonmagnetic electrically conductive spacer layer, which is typically copper (Cu). One ferromagnetic layer has its magnetization direction fixed, such as by being pinned by exchange coupling with an adjacent antiferromagnetic layer, and the other ferromagnetic layer has its magnetization direction “free” to rotate in the presence of an external magnetic field. With a sense current applied to the sensor, the rotation of the free-layer magnetization relative to the fixed-layer magnetization is detectable as a change in electrical resistance.
In a magnetic recording disk drive SV read sensor or head, the stack of layers are located in the read “gap” between magnetic shields. The magnetization of the fixed or pinned layer is generally perpendicular to the plane of the disk, and the magnetization of the free layer is generally parallel to the plane of the disk in the absence of an external magnetic field. When exposed to an external magnetic field from the recorded data on the disk, the free-layer magnetization will rotate, causing a change in electrical resistance. If the sense current flowing through the SV is directed parallel to the planes of the layers in the sensor stack, the sensor is referred to as a current-in-the-plane (CIP) sensor, while if the sense current is directed perpendicular to the planes of the layers in the sensor stack, it is referred to as current-perpendicular-to-the-plane (CPP) sensor.
CPP-SV read heads are described by A. Tanaka et al., “Spin-valve heads in the current-perpendicular-to-plane mode for ultrahigh-density recording”, IEEE TRANSACTIONS ON MAGNETICS, 38 (1): 84-88 Part 1 January 2002. Another type of CPP sensor is a magnetic tunnel junction sensor, also called a tunneling MR or TMR sensor, in which the nonmagnetic spacer layer is a very thin nonmagnetic tunnel barrier layer. In a CPP-TMR sensor the tunneling current perpendicularly through the layers depends on the relative orientation of the magnetizations in the two ferromagnetic layers. While in a CPP-SV read head the spacer layer is formed of an electrically conductive material, such as Cu, in a CPP-TMR read head the spacer layer is formed of an electrically insulating material, such as TiO2, MgO or Al2O3.
The CPP MR sensors described above are “single” CPP sensors because they have a single spacer layer and a single pinned layer. A “dual” CPP MR sensor has a second spacer layer and a second pinned layer located on the other side of the free layer, i.e., the side opposite the first spacer layer and first pinned layer. Dual CPP MR sensors are well-known in the art and have been proposed to provide an increased magnetoresistance (ΔR/R) over single CPP MR sensors. Single CPP MR sensors are known to be susceptible to current-induced noise and instability. The spin-polarized current flows perpendicularly through the ferromagnetic layers and produces a spin transfer torque on the local magnetization. This can produce continuous gyrations of the magnetization, resulting in substantial magnetic noise if the sense current is above a certain level. It has been demonstrated that dual CPP-SV sensors may reduce the sensitivity of the free layer to spin-torque-induced instability. (J. R. Childress et al., “Dual current-perpendicular-to-plane giant magnetoresistive sensors for magnetic recording heads with reduced sensitivity to spin-torque-induced noise”, J. Appl. Phys. Vol. 99, 08S305, 2006).
The free layers in both single and dual CPP sensors often exhibit magnetostriction, which is the property of a ferromagnetic material that causes it to change its shape when subjected to a magnetic field. In magnetic recording sensors, magnetostriction is also to be avoided because a strain produced by residual stresses in the sensor results in an undesireable uniaxial magnetic anisotropy in the sensor that can interfere with its expected magnetic performance. Moreover, to optimize sensor magnetic stability, positive magnetostriction is to be avoided in CPP sensor free layers.
One of the challenges in the design of CPP-MR sensors is to achieve high magnetoresistance while keeping the magnetic moment of the free layer below a target value and the magnetostriction of the free layer near zero or less than zero. What is needed is a CPP MR sensor with an improved free layer structure that achieves these objectives and that has reduced sensitivity to spin-torque-induced instability.